1. Field of the Invention
The present invention relates to a thermosensitive recording material which causes color development by the application of heat thereto, thereby forming images thereon.
2. Discussion of Background
Conventionally, a thermosensitive recording material is prepared by forming a thermosensitive coloring layer on a support such as a sheet of paper or synthetic paper, or a plastic film. The thermosensitive coloring layer comprises as the main component a thermosensitive coloring composition, so that the thermosensitive recording material is capable of producing a colored image by the application of heat thereto using a thermal head, thermal pen, laser beam, or stroboscopic lamp.
This type of thermosensitive recording material has wide-scale utilization, not only as a recording material for copying books and documents, but also as a recording material for use with printers for electronic calculators, facsimile machines, ticket vendors, and label recorders because of the following advantages over other conventional recording materials:
(1) image recording can be speedily performed, using a comparatively simple device without complicated steps for development and image fixing; PA1 (2) the thermosensitive recording material can be produced and used without generating noise and causing environmental pollution; and PA1 (3) the manufacturing cost of the thermosensitive recording material is low.
The thermosensitive coloring composition for use in the thermosensitive recording material comprises a coloring agent and a color developer capable of inducing color formation in the coloring agent upon application of heat thereto. Conventionally, colorless or light-colored leuco dyes having a lactone, lactam, or spiropyran ring, are employed as the coloring agents; and organic acids and phenolic materials are employed as the color developers. This kind of thermosensitive recording material comprising the aforementioned leuco dye and color developer in combination is widely utilized because the color tone of the obtained images is clear, the whiteness degree of the background of the thermosensitive recording material is high, and the obtained images have excellent weather-resistance.
Recently, the demand for a relatively thick thermosensitive recording material has greatly expanded because this kind of thick thermosensitive recording material has been applied to a tag, ticket, card or price tag.
In a conventional recording apparatus used for the thermosensitive recording material, images are formed on the thermosensitive recording material while the recording material is transported by a platen roller, with the recording material being caused to closely adhere to a thermal head by the application of pressure to the recording material toward the thermal head using pressure-application means such as a spring.
To improve the adhesion between the thermosensitive recording material and the thermal head, the thermosensitive recording material is required to have a proper surface smoothness and flexibility. The adhesion of the thermosensitive recording material to the thermal head is improved by increasing the surface smoothness of the recording material, as disclosed in Japanese Patent Publication 52-20142 and Japanese Laid-Open Patent Application 54-115255.
The problem of poor adhesion between the thermosensitive recording material and the thermal head can be solved by improving the surface smoothness of the recording material in the case where a support for use in the recording material is as thin as a sheet of plain paper. In contrast to this, when the thermosensitive recording material comprising a relatively thick support material is employed, which will be hereinafter referred to as a thick thermosensitive recording material, it is impossible to improve the adhesion to the thermal head and cope with the high-speed printing operation merely by increasing the surface smoothness of the recording material. The adhesion of the conventional thick thermosensitive recording material to the thermal head is poor because the stiffness of the thick thermosensitive recording material is too large. As a result, image unevenness and blurring occur in the course of thermal printing. Thus, the conventional thick thermosensitive recording material cannot produce high quality images.